Other Intangible Assets and Goodwill - Net Carrying Value of Core Deposit Intangibles (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Core Deposit Intangibles and Aggregate Amortization Expense
Net carrying value	$ 55,509	$ 63,286
Aggregate amortization expense	10,370	8,485	$ 6,270
Core Deposits Intangibles
Net Core Deposit Intangibles and Aggregate Amortization Expense
Gross carrying value	88,099	85,506	62,977
Accumulated amortization	(32,590)	(22,220)	(13,735)
Net carrying value	55,509	63,286	49,242
Aggregate amortization expense	10,370	$ 8,485	$ 6,270
Estimated Amortization Expense
For the year ending December 31, 2021	9,951
For the year ending December 31, 2022	9,414
For the year ending December 31, 2023	8,614
For the year ending December 31, 2024	7,826
For the year ending December 31, 2025	$ 6,750

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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